1. Field of the Invention
The present invention relates to a linear High Voltage Power Supply (HVPS) and its filters for high power RF transmitting tubes, such as Inductive Output Tubes (IOTs) that may be employed in a cost effective amplifier suitable for use, for example, in a digital television transmitter for the broadcast industry, or in any other appropriate application for such an amplifier.
2. Discussion of the Background
In Broadcast Television, transmitters for UHF frequencies typically require much higher RF power (energy) than VHF transmitters. Typically, tubes employed in UHF transmitters have outputs of 20-30 kW. For UHF transmitters, the Inductive Output Tube (IOT) is usually the device best suited for high power amplification. The IOT is, however, easily damaged internally from high voltage arcs that can occur inside the vacuum envelope of the tube. The damage is primarily caused by the release of stored energy from the filter circuit of the High Voltage Power Supply (HVPS). The output of the HVPS may be 20-40 kV, at 2-3 A, so the stored energy can be considerable. Also contributing significantly is the power that is still available from the input AC power to the HVPS until the AC mains can be interrupted after an arc starts. Traditional IOT amplifiers utilize electromechanical contactors to connect and interrupt the input AC power. These contactors can take between 30 and 50 milliseconds to interrupt the AC.
When the very first IOT amplifier devices in UHF television transmitters became commercially available in 1988 to replace klystrons, one significant difference between these tubes and the older klystron technology was the requirement for fast removal of the high voltage, in the event of an arc within the vacuum envelope of the tube, to limit the release of stored energy enough to prevent any permanent internal damage to the IOT. For example, for IOTs commonly available from Marconi Applied Technologies, it is specified that this energy should not exceed 20 Joules. For analog television broadcasting, considerable stored energy in the power supply filter, especially in capacitors, was required to handle the signal to noise requirements and the long periodic duration, dynamic load changes of the analog signal on the high voltage power supply.
The common method to use in accomplishing this fast removal of high voltage was to use a crowbar circuit incorporating a triggered spark gap or a hydrogen thyratron, which protects the IOT by shunting the energy of the power supply. As technology has progressed, there have been instances where the use of a switching power supply, with low inherent stored energy coupled with a high speed switching regulator circuit, could provide proper IOT internal arc protection. Both the crowbar and the switching power supply are viable, industry standard solutions, but come with an associated cost and complexity.
The most economical and reliable HVPS is the linear type, which consists of a transformer, a full-wave rectifier and a filter, utilizing the AC power line frequency. Because of the low frequency, filter components have high values and consequentially can store large amounts of energy. To accomplish fast removal of high voltage, transmitter manufacturers have utilized crowbar circuits incorporating devices such as triggered spark gaps and hydrogen thyratrons to shunt the energy of the power supply around the IOT, as already mentioned. The operation of the crowbar circuit can cause very high current surges both in the high voltage power supply (HVPS) as well as in the AC line voltages supplying the transmitter. The high AC current surges can cause excessive wear and/or burning of the switch contacts in the contactors and circuit breakers that feed the power supply and can cause glitches or transients on the AC power lines that can effect other equipment operating nearby.
A medium to high frequency switching regulator type power supply, because of its higher frequency and the nature of the electronics that drive the xe2x80x9cswitchingxe2x80x9d, can provide an HVPS with low stored energy and a fast switch-off of the input power, thus eliminating the requirement for a shunt type crowbar system.
Both the shunt type crowbar and the switching type HVPS add complexity and reliability issues to the amplifier, as well as additional costs.
Prior to the present invention, the state of the art has generally been considered to be that either a switching type HVPS was required to eliminate the need for a crowbar circuit, or that if a linear HVPS was used, then a crowbar circuit had also to be used.
The above assumptions made in the prior art were based upon accommodating the needs of an analog television transmission system. The broadcast industry is transitioning from analog to digital, and the digital (DTV) transmitters have a lower Signal-to-Noise Ratio (SNR) requirement. The DTV signal also presents a different characteristic for the dynamic load change to the HVPS.
The generally accepted standard for measuring the potentially damaging, stored energy an IOT can be subjected to by the HVPS system is the xe2x80x9cwire test.xe2x80x9d This test is described as putting a specified length and size of fine wire between the power supply and the load, then causing a short circuit around the load and seeing if the wire is damaged or burned up before the high voltage is removed from the load. For example, a wire test published by Marconi Applied Technologies requires that 300 mm length of 36 AWG wire shall not fail when tested as described above. Thales Electron Devices, on the other hand, specify that the enamel should not be damaged on 375 mm length of 34 AWG wire. Other manufacturers of IOTs have published their own specific variation of a wire test; details of these are readily available in the particular data sheets or user guides.
The traditional filter shown in FIG. 1 was designed to have an amplitude of hum, ripple and noise to be at least 60 dB below the level of the high voltage. This filter has no added series resistance to the inductor, and thus has no current limiting effects until the AC mains are interrupted (follow-on or follow-through current.) The capacitor in this filter is typically 8 xcexcF for analog service, whereas for DTV, the capacitance can be much less. With an appropriate resistance in series with an appropriately sized capacitor, energy from the capacitor can be adequately limited, but the follow-through and stored energy in the inductor is not addressed.
In FIG. 1, a filter circuit that is typical of the conventional art is shown. Input power is delivered from an AC source 11, typically 480 v three-phase, via a switch 10 to a transformer and rectifier block 1. An inductor 3 is in series with the output of the transformer and rectifier block 1 and the input of a load device 2. A capacitor 4 (having a typical value of 8 microfarads) usually has a resistor 5 (having a typical value of 60 Ohms) arranged in series therewith to provide charge current limiting, and ripple current limiting for the capacitor 4. The resistor 5 also limits the current from the capacitor 4, but not the inductor 3, during a short circuit or high voltage arc event. Such an event is detected by excess current in current transformer 8 operating a crowbar 9 to shunt the HVPS output and open the switch 10.
One aspect of the present invention is to address and resolve the above-identified and other limitations of background art devices.
This invention is particularly, but not exclusively applicable to digital television transmitters and CW (continuous wave) or pulsed RF amplifiers where a signal to noise ratio requirement is not as stringent as in an analog television transmitter. In such applications, this system design can leverage the less stringent filtering requirements of the HVPS, to develop a transmitter amplifier system that exploits the lower cost of the linear HVPS and eliminates the cost and complexity of either a shunt crowbar or a switching power supply. A solid state type switch for the AC mains is used for its faster turn off time, even though it adds some additional cost and complexity. A solid-state switch using an SCR device can interrupt the AC supply to the transformer in approximately 9 milliseconds when excessive load current is detected. This type of device is required to appropriately limit the follow-on current. Other more exotic solid state switching devices and circuits that operate even faster are alternatives as well.
This invention addresses the stored energy in the HVPS as well as the speed at which the AC line is opened up (follow-on current) to eliminate the need for the crowbar circuit. The filter in the HVPS is important to the performance of the transmitter and therefore cannot be discarded. The invention includes a filter that maintains the performance of the transmitter while reducing the stored energy and/or limiting the discharge rate of the stored energy thereby creating a system that not only will meet the requirements of the wire test but will also protect an IOT from damage caused by an arc within the vacuum envelope.
This invention provides a solution to the problems discussed in the background art by way of a system that utilizes a xe2x80x9cstandardxe2x80x9d type linear high voltage power supply, a solid state, electronic primary switch to facilitate the removal of the input AC mains power faster than the typical electromechanical contactor, and an output filter on the power supply that has a low enough stored energy, but sufficient filtering for the DTV (digital television) signal. The DTV signal provides a benefit for this application in that it has a lower signal to noise (SNR) ripple requirement from the HVPS and experiences much shorter duration, dynamic load changes than analog television. A filtered linear HVPS according to the present invention is arranged in such a manner as to properly provide power to an IOT used in DTV service while fully protecting the IOT from potential harm due to high voltage arcs, without the use of either a protective shunt crowbar system, or a medium to high frequency switching regulator type power supply. The filter meets DTV performance requirements and protects an IOT in a manner that meets the IOT manufacturer""s xe2x80x9cwire testxe2x80x9d requirements.
Moreover, a feature of the invention is to take the protection requirements imposed by the manufacturers of the IOTs and the SNR requirement of the amplifier system to develop a filter system for a linear HVPS that results in an IOT amplifier that uses a linear HVPS without a crowbar circuit.